Swagger paths without operationId property should not crash connexion

[hjacobs]

To ease incremental development, Connexion should not fail if one of the operationId properties is missing.

$ python3 hack.py
Traceback (most recent call last):
File "hack.py", line 4, in
app.add_api('swagger.yaml')
File "/usr/local/lib/python3.4/dist-packages/connexion/app.py", line 81, in add_api
api = connexion.api.Api(yaml_path, base_path, arguments, swagger_ui)
File "/usr/local/lib/python3.4/dist-packages/connexion/api.py", line 73, in init
self.add_paths()
File "/usr/local/lib/python3.4/dist-packages/connexion/api.py", line 106, in add_paths
self.add_operation(method, path, endpoint)
File "/usr/local/lib/python3.4/dist-packages/connexion/api.py", line 90, in add_operation
security_definitions=self.security_definitions)
File "/usr/local/lib/python3.4/dist-packages/connexion/operation.py", line 52, in init
self.operation_id = operation['operationId']
KeyError: 'operationId'